Method of and apparatus for the prevention of surging with axial compressors



March 15, 9 J. PETTERSEN ETAL 3,240,422

METHOD OF AND APPARATUS FOR THE PREVENTION OF SURGING WITH AXIALCOMPRESSOHS Filed March 27, 1965 2 Sheets-Sheet 1 f Z gzg I h GOA/$77INVENTORS Johan Pe'kt ersen Hans Baumcmh /iw, JWN Y ANA ATTORNEY S March15, 1966 .1. PETTERSEN ETAL 3,240,422

METHOD OF AND APPARATUS FOR THE PREVENTION OF SURGING WITH AXIALCOMPRESSORS Filed March 27, 1963 2 Sheets-Sheet 2 3 69 INVENTOR S Jul1cm Peitersen Hans Baumam n ATTORNEYS United States Patent 3,240,422METHOD OF AND APPARATUS FOR THE PREVENTION OF SURGING WITH AXIALCOMPRESSORS Johan Pettersen, Baden, and Hans Baumann, Nussbaumen,Aargau, Switzerland, assignors to Aktiengesellschaft Brown, Boveri &Cie, Baden, Switzerland, a joint-stock company Filed Mar. 27, 1963, Ser.No. 268,306 Claims priority, application Switzerland, Apr. 3, 1962,

4,048/62 Claims. (Cl. 230-415) The present invention relates to animproved method for the prevention of surging with single or multi-stageaxial compressors and to an apparatus for putting this method intopractice.

Should the back pressure of an axial compressor be gradually increasedthen the output quantity decreases in accordance with a certain law. Thereference point of the operating condition sochanging describes a curveon the pV diagram, the so-called characteristic of the compressor at thespeed concerned. On reaching a certain pressure the compressor, whichhas up till now been running smoothly, now begins to operate in anirregular manner and considerable pulses and oscillations occur in theflow. This unstable manner of working of a compressor is termed surging?and the collection of all points at which the phenomenon appears isdescribed as the surge limit.

In some cases of operation, even on approaching the surge limit therearises a disturbance in the flow, this being designated as rotatingstall. In all cases, however,

violent aperiodic shocks occur just before reaching the surge limit aswell as in the surge zone itself. These impulse disturbances stimulateoscillations in the blading concerned, the former being capable ofassuming such proportions that danger of breakage arises.

To prevent surging and the blade vibrations caused by it, it. is a knownprocedure to employ a surge prevention regulator which opens a blow-offvalve as seen as the operating point approaches the critical zone. Bythis means the output quantity is increased and the compressor can oncemore operate in a quiet and stable fashion. With the present-day surgeprevention regulators-one of this type is indicated in FIG. 3 with 16and described in the accompanying text-a function is formulated of thesuction volume and the end pressure or pressure at an intermediate stagewith the aid of a cam, this then. acting as input for the regulator.Nonetheless, these regultors have a considerable disadvantage:

On the commissioning of a new compressor, the surge limit a (FIG. 1) isfixed by experiment. A line b is now chosen at an appropriate distancefrom this limit, this acting as the limit of approach for thesurge-prevention regulator. As soon as the fluctuating operating point Preaches this limit, the regulator comes into action. In this way it isprevented that the operating point goes as far as the surge limitwhereby quiet operation of the compressor should be guaranteed under alloperating conditions. Now, however, surge limit a can displace itself inthe direction of the limit of approach b of the regulator, e.g. due tocontamination of the blading or change in the gas inlet temeprature, sothat the safety factor becomes too small. As the limit of approach isde- "ice termined on the basis of measurements at commissioning andthereafter remains constant, it can happen that the compressor begins tosurge despite perfect functioning of the regulation.

This disadvantage of known regulation apparatus is avoided according tothe invention by measuring the vibrations of the guide and/ or rotorblades and using the greatest determined amplitude of vibration as theparameter of regulation for the purpose of preventing surgmg.

In FIGS. 2 and 3 are represented schematically two constructionalexamples of the invention, whereby like components are denoted by thesame reference symbols. Detail views of them are given in FIGS. 4 and 5.

According to FIG. 2, the axial compressor 1 isdriven by motor 2, drawsgas in from connection line 3 and passes it thru outlet connection line4. Vibration feelers 6 are mounted on the guide blades 5, the impulsesfrom the former being transmitted as an input parameter to regulator 7via an amplifier 8. As soon as the amplitude of the blade vibrationsexceeds a maximal permissable valuewhich is declared by the proximity ofthe surge limit--the regulator is acutated. It opens an outflow 9through which control oil runs out of the pressure system 10, the feed11 to which is adjusted by orifice 12.

As soon as the oil pressure in system 10 sinks, control piston 13 of theservo-motor 14 opens the blow-off valve 15 from output line 4 and thusmakes an additional outlet available to the compressed gases. The outputvolume becomes greater once more, the operating point of the compressorfalls back into the region of stable flow because of thisto the right inFIG. 1-and the Surging is prevented. If the blade vibrations fade awayagain, regulator 7 closes off the outflow 9, the pressure in system 10increases and piston 13 of servo-motor 14 shuts the blow-oil valve 15.

FIG. 3 shows another possible application of the apparatus as accordingto the invention. Axial compressor 1, motor 2, line connections 3 and 4,outflow 9, pressureoil system 10, oil feed 11, orifice 12, controlpiston 13 of the servo-motor 14 and the blow-off valve 15 have the samefunction as in FIG. 2. The outflow 9 together with the blow-off valve 15are now, however, actuated by a surge-prevention regulator 16 which isalready given in the introduction to the description as being known. Itis commented on only briefly here.

The surge prevention regulator is provided with a pressure differencemeasuring apparatus 17 on the membrane 18 of which acts the pressuredifierence at the throughput measuring point 19 in the suctionconnection 3. Should the throughput quantity or, respectively thepressure difference at the measuring point 19 decrease to a certainminimum value, then the force exerted upwards by the membrane 18 fallsbelow the pretension of spring 20 which now moves the oil blow-off valve21 to the open position. As soon as oil runs out through the outflow 9,the pressure falls in the hydraulic system 10 which then causes theopening to blow-01f valve 15.

The characteristic value of the regulator 16, ie the minimum pressure.difference at measuring point 19 at which the membrane 18 opens the oilblow-0E valve 21, is thus determined by the pretension in the spring 20.This pretension can be altered by displacing cam 22 which is set duringthe commissioning and which can be corrected by hand with the aid ofadjustment spindle 23 in later service. During operation, however, thisresults by means of the bellows element 24 which makes the setting as afunction of the pressure in pressure line 4. For example, should theoutput pressure increase, then the bellows is pressed together againstthe force of the adjustment spring 25 thereby displacing the cam to theleft. The form of the latter is chosen so that on displacement to theleft, the pretension of spring 20 is raised. This means, however, thatthe oil blow-off valve 21 now opens at a greater pressure difference onthe membrane 18. Otherwise expressed: the greater the output pressure isin line 4, the greater too is the minimum suction quantity at which theregulator is actuated. The form of the cam 22 permits the limit ofapproach b of the regulator to be adapted to the surge limit a of thecompressor with any desired exactness (see FIG. 1).

The surge-prevention regulator 16 and that part of the installation asgiven in FIG. 3 and as described earlier are known and are frequentlyemployed in this arrangement. It will now be shown that the apparatus asaccording to the invention may also be applied with advantage for thispurpose.

As in FIG. 2, vibration feelers 6 are also attached to the guide blades5, these feelers serving to transmit the blade vibrations as impulses tothe regulator 7 via amplifier 8. Here, however, the regulator does notact directly on the blow-off valve 15 any more. When the vibrations ofthe blades become too great and thus when the operating point is drawingtoo close to the surge limit without the surge-prevention regulatoroperating, then regulator 7 acts on the adjustment motor 26 whichactuates the adjustment spindle 23. Cam 22 thus travels to the left andopens the oil blow-off valve 2 1, this then causing an opening ofblow-off valve 15, as is described above. The pressure in the outputline 4 and at the throughout measuring point 18 remain at the same time,or leastwise at first, unchanged. By changing the position of the cam22, the function between output pressure and suction quantity was thusaltered. This means that the regulator 7 has displaced the limit ofapproach b (FIG. 1) of the surgeprevention regulator 16 and therebyadapted it to the new, actual surge limit.

In the constructional examples in FIGS. 2 and 3, vibration feelers aremounted on each row of guide blades but in fact the moving blades couldequally as well be called upon for this. Admittedly, there arise certaindifiiculties in leading out the connecting cables from the rotor withthis arrangement. In principle it is suflicient merely to mount avibration feeler on only one guide or moving blade. Naturally, one wouldchoose for that purpose the row of blades at which surging first occursas given by calculation or experience. But as this can be various rowsof blades depending on the changing operating conditions, it is usual toattach a vibration feeler to one blade of each endangered row.Nonetheless, the amplifier accepts only the greatest amplitude ofvibration at any one time and passes this on to the regulator whichcomes into action as a limit regulator only from a predetermined minimumvalue onwards.

As vibration feeler there are various known types of equipment atdisposal. In FIG. 4 is shown the application of a strain gauge 27 ofwhich one is attached to moving blade 28 and another to guide blade 5.Preferably, the strain gauges are fixed to the feet of the blades as thegreatest material stresses occur there due to the blade vibrations andare thus the easiest to measure. The connecting'cables 29 lead toamplifier 8 which is not drawn in here.

FIG. shows the application of vibration guages 30. For this there may beconsidered such appliances which measure the amplitude, the velocity orthe acceleration of the blade vibrations. These types of apparatus arebest built into the blade tips as the blade amplitude is greatest themeand the measurement most effective.

4 Should it be feared that disturbances may arise in the flow due .tothe apparatus itself or due to the connecting cables 29, then thevibration gauges may also be mounted on the blade foot; in this case,however, more sensitive apparatus and a higher amplification of theimpulses are then required.

As was said above, the vibration feelers can be mounted on either one oron several rows, and both on the guide and moving blades, as isrepresented in FIGS. 4 and 5, or only on one of the two. If it is to beavoided that the connection cables be led out of the rotor, whereas, onthe other hand it is a row of moving blades which stands in the greatestdanger, then a possible advantageous method of measurement is presentedif the natural frequency of the moving blade to be measured coincideswith the natural frequency of the neighboring guide blade. In this casethe vibrations of the moving blade are transferred synchronously on tothe guide blade and can be measured there more simply.

The regulator 7 can act in various ways on the compressor. Arrangementsare thus known with which the regulator actuates a component other thanthe blow-off valve or the cam of the surge-prevention regulator. It mayclearly be understood that corresponding arrangements can also beeffected with a regulation as according to the present invention.

Furthermore, the apparatus can also serve only for surveillance so that,should the amplitude of the blade vibrations exceed an allowable value,the impulse from the vibration feeler releases an acoustic or visualsignal whereupon the necessary control measures may result from hand.

The apparatus can, of course, be employed to check blade vibrations ofturbines, e.g. at start up whereby the regulator operates naturally onanother control organ, preferably the inlet valve.

By means of the described method and the appertaining apparatus, surgingof axial compressors is avoided. The regulation does not take effect onreaching some fixed line in the pV-diagram, but rather on approachingthe effective surge limit so that any displacement of the latter istaken into consideration without correction by hand being necessary. Theamplifier can be so set that the regulator comes into action even beforethe occurrence of those amplitudes of vibration which would cause damageof the blades.

We claim.

1. In an axial flow compressor having at least one stage comprising arow of stationary guide blading and an adjacent row of blading mountedon the compressor rotor, the method of preventing said compressor fromsurging which comprises the step of measuring vibrations induced in theblading as a result of fluid flow therethrough and utilizing only suchmeasured vibrations as reach a predetermined amplitude for regulatingfluid flow through the compressor in such sense as to reduce itstendency to surge.

2. Apparatus for preventing surging in an axial flow compressor havingat least one stage comprising a row of stationary blading and anadjacent row of blading mounted on the compressor rotor comprising, afeeler element in direct association with said blading, said feelerelement producing a signal proportional to the amplitude at which saidblading vibrates, and means responsive only when said signal reaches apredetermined level for regulating fluid flow through said compressor insuch sense as to reduce its tendency to surge.

3. Apparatus as defined in claim 2 for preventing compressor surgewherein said feeler element is applied to said guide blading and thenatural frequency of said guide blading matches that of said movableblading.

4. Apparatus as defined in claim 2 for preventing compressor surgewherein said compressor has a plurality of stages and a vibration feelerelement is provided for the blading of several stages, the signalsproduced by said feeler elements being transmitted to the input of anamplifier and said amplifier producing at its output a control signalregulating fluid flow through said compressor whenever the signalproduced by any one of said feeler elements reaches said predeterminedlevel.

5. Apparatus for preventing surging in an axial flow compressor havingat least one stage comprising a row of stationary blading and anadjacent row of blading mounted on the compressor rotor, meansresponsive to a predetermined decrease in pressure obtained across ameasuring point of said compressor for regulating fluid flow throughsaid compressor in such sense as to reduce its tendency to surge, meansfor adjusting the pressure at which said regulating means becomesoperative, a feeler element in direct association with said blading,said feeler element producing a signal proportional to the ReferencesCited by the Examiner UNITED STATES PATENTS 2,395,995 3/1946 Dewey 230172,424,137 7/1947 Ball 230115 2,455,292 11/1948 Borden 230-1 15 2,696,34512/1954 Hopper 2301 14 3,058,339 10/1962 Shapiro 737 1.4

15 LAURENCE V. EFNER, Primary Examiner.

1. IN AN AXIAL FLOW COMPRESSOR HAVING AT LEAST ONE STAGE COMPRISING AROW OF STATIONARY GUIDE BLADING AND AN ADJACENT ROW OF BLADING MOUNTEDON THE COMPRESSOR ROTOR, THE METHOD OF PREVENTING SAID COMPRESSOR FROMSURGING WHICH COMPRISES THE STEP OF MEASURING VIBRATIONS INDUCED IN THEBLADING AS A RESULT OF FLUID FLOW THERETHROUGH AND UTILIZING ONLY SUCHMEASURED VIBRATIONS AS REACH A PREDETERMINED AMPLITUDE FOR REGULATINGFLUID FLOW THROUGH THE COMPRESSOR IN SUCH SENSE AS TO REDUCE ITSTENDENCY TO SURGE.